1. Field of the Invention
The present invention relates to a multistage compression refrigerating machine such as a centrifugal chiller, screw chiller, or the like.
2. Description of the Related Art
Multistage compression refrigerating machines are widely used in air conditioning systems of general buildings, factories, and the like. For example, the two-stage compression refrigerating machine as shown in FIG. 3 comprises an evaporator 51, a first-stage compressor 53 and a second-stage compressor 54 which are rotationally driven by an electric motor 52 (abbreviated to the motor 52, hereinbelow), a condenser 55, a subcooler 56, a motor cooler 57 for cooling the motor 52 by using a refrigerant, and a lubricating-oil cooler 58 for cooling lubricating oil by using a refrigerant.
In the evaporator 51, a liquid refrigerant is heated by cold water 60 having a temperature of 12xc2x0 C. passing through a tube 59, so that vaporized refrigerant 61 is generated. In this process, the cold water 60 is cooled to approximately 7xc2x0 C. thorough the heat exchange in the evaporator 51, and it is then delivered outside. Therefore, the temperature in the evaporator 51 is maintained to be approximately 5xc2x0 C.
The vaporized refrigerant 61 generated in the evaporator 51 is supplied to the first-stage compressor 53 and second-stage compressor 54, and the supplied refrigerant is two-stage-compressed by using impellers which are rotated by the motor 52, thereby discharging high-temperature and high-pressure vaporized refrigerant 61a. Here, vaporized refrigerant 61b from the subcooler 56 is also introduced (or supplied) into a path between the first-stage and second-stage compressors 53 and 54 (i.e., the upstream side of the second-stage compressor 54), and the supplied vaporized refrigerant 61b is also compressed together with the vaporized refrigerant 61 from the evaporator 51.
In the condenser 55, the high-temperature and high-pressure vaporized refrigerant 61a discharged from the second-stage compressor 54 is cooled using cooling water 63 which flows through a tube 62, thereby condensing the vaporized refrigerant 61a into a liquid. In this process, the cooling water 63 is heated through the heat exchange in the condenser 55 and is then discharged outside. The condensed liquid refrigerant 64 is collected at the bottom of the condenser 55; thus, the temperature inside the condenser 55 is approximately 40xc2x0 C.
The pressure of the liquid refrigerant 64a supplied from the condenser 55 is reduced to an intermediate pressure by using a first-stage expansion valve 65, so that the refrigerant 64a is expanded, and a portion of the expanded refrigerant is output from the subcooler 56 as vaporized refrigerant 61b. As explained above, this vaporized refrigerant 61b is supplied to an intermediate position between the first-stage compressor 53 and the second-stage compressor 54. On the other hand, the pressure of the remaining refrigerant 64a cooled through the evaporation of the refrigerant 64a is further reduced using a second-stage expansion valve 66 and is then supplied to the evaporator 51.
In addition, a portion 64b of the refrigerant 64, which is collected at the bottom of the condenser 55, is used for cooling the motor 52 and the lubricating oil. More specifically, the refrigerant 64b is first supplied to the lubricating-oil cooler 58 so as to cool the lubricating oil and is then supplied to the motor cooler 57 so as to cool the motor 52. After that, the refrigerant 64b including a vaporized portion is returned to the evaporator 51.
However, in the conventional multistage compression refrigerating machines, the refrigerant 64b (a portion of the liquid refrigerant 64) collected at the bottom of the condenser 55 having a temperature of approximately 40xc2x0 C. is used for cooling the motor 52 and the lubricating oil, and the refrigerant 64b after the cooling process is returned to the evaporator 51 whose inner temperature is approximately 5xc2x0 C. Therefore, the liquid refrigerant 64b expands due to a pressure difference between the condenser 55 and the evaporator 51, and as a result, the refrigerant 64b evaporates in the evaporator 51. Accordingly, the amount of the liquid refrigerant to be used to provide or increase the refrigerating capacity is reduced, thereby decreasing the refrigerating capacity.
In consideration of the above circumstances, an object of the present invention is to provide a multistage compression refrigerating machine for efficiently cooling a rotating machine such as an electric motor and lubricating oil by using a refrigerant and increasing the amount of refrigerant to be used to provide the refrigerating capacity in the evaporator, thereby improving the refrigerating capacity.
Therefore, the present invention provides a multistage compression refrigerating machine comprising:
an evaporator;
a condenser for condensing a refrigerant and supplying the condensed refrigerant to the evaporator via a subcooler:
a multistage compression system having a plurality of compressors which are connected in series, for:
receiving the refrigerant evaporated in the evaporator;
receiving a refrigerant evaporated from the subcooler, from an intermediate position between adjacent compressors in the multistage compression system; and
compressing the received refrigerants together and discharging the compressed refrigerant to the condenser;
a rotating machine for driving the multistage compression system;
a rotating-machine cooler for cooling the rotating machine; and
a lubricating-oil cooler for cooling lubricating oil for lubricating the rotating machine, and wherein:
the refrigerant extracted from the subcooler is supplied to the rotating-machine cooler and the lubricating-oil cooler, and this refrigerant is returned to the evaporator after cooling.
According to the present invention, the rotating machine and the refrigerant can be efficiently cooled, and the amount of the liquid refrigerant (in the evaporator) to be used to provide or increase the refrigerating capacity can be reduced, thereby improving the refrigerating capacity and reducing the running cost.
It is possible that:
one or more subcoolers connected in series are provided for supplying the evaporated refrigerant from each subcooler to each intermediate position between adjacent compressors of the multistage compression system; and
the refrigerant supplied to the lubricating-oil cooler and the rotation-machine cooler is extracted from the subcooler positioned at a position most downstream of the subcoolers connected in series.
In this case, the refrigerant capacity can be further improved and the cost can be further reduced.
Typically, the rotating machine is an electric motor.